The term “zeolite” was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that had previously been adsorbed into the material. Based on this, he called the material zeolite, from the Greek zeo, meaning “boil” and lithos, meaning “stone”.
We now know that zeolites are microporous, aluminosilicate or silicate minerals. As of November 2010, 194 unique zeolite frameworks have been identified (DDR being one of them), and over 40 naturally occurring zeolite frameworks are known. Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite.
Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+, and many others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution.
The regular pore structure and the ability to vary pore size, shape and chemical nature makes zeolites very useful as molecular sieves. Depending on their structure and composition, zeolites can separate molecules based on adsorption and/or diffusion of certain molecules preferentially inside the pores or exclusion of certain molecules based on their size. The pore size is typically less than 2 nm and comparable to that of small molecules, allowing the use of zeolites to separate lightweight gases such as CO2 and CH4.
The maximum size of a species that can enter the pores of a zeolite is controlled by the dimensions of the channels in the zeolite. These are conventionally defined by the ring size of the aperture, where, for example, the term “8-ring” refers to a closed loop that is built from 8 tetrahedrally coordinated silicon (or aluminum) atoms and 8 oxygen atoms. The rings are not always symmetrical due to a variety of effects, including strain induced by the bonding between units that are needed to produce the overall structure, or coordination of some of the oxygen atoms of the rings to cations within the structure. Therefore, the pores in many zeolites are not cylindrical. The DDR (aka Deca-dodecasil 3R) zeolite of this invention has an 8-ring pore structure (see FIG. 1).
Thus, zeolites are widely used in industry for water purification, as petrochemical production catalysts, and in nuclear reprocessing. Their biggest use by volume is in the production of laundry detergents, and they are also used in medicine and in agriculture.
In particular, zeolites have been used in two types of molecular sieving membranes: mixed matrix membranes and pure zeolite membranes. To fabricate a mixed matrix membranes, zeolite crystals are first dispersed in a polymer solution. The dispersion is then cast into a film or spun into a tubular hollow fiber. Since the membrane thickness is desired to be less than 1 micron, it is necessary to have submicron zeolite particles.
In pure zeolite membrane fabrication, zeolite crystals are first deposited as a “seed” coating on a porous substrate and then grown into a thin continuous layer known as a zeolite membrane. The porous substrate provides mechanical stability for the membrane. In this approach crystals with submicron size are also preferred because the seed coatings will then be tightly packed and of high quality. Further, membrane thickness is ideally about 0.5-5 μm.
Among the various zeolite materials, DDR is a pure silica (SiO2) zeolite. The pore system comprises relatively large (19-hedral) cages interconnected through 8-ring windows with aperture approximately 3.6×4.4 Å. Due to its relatively small pore size, DDR can be used to separate light gases, such as CO2 (kinetic diameter=3.3 angstroms) from CH4 (diameter=3.8 angstroms). Other advantages of DDR zeolites include high thermal stability and chemical resistance due to the pure silica composition.
DDR zeolite crystals were first synthesized in 1986 and the synthesis was further developed by several researchers. These synthesis methods either take a long time (9-25 days) or produce very large crystals (20 to 50 micrometers).
With the exception of a DDR membrane investigated for H2/CO2 separation by Zheng [21] and a DDR membrane for investigation of gas diffusion at high temperature by Kanezashi [22], to date the DDR membranes reported in the literature are those synthesized by researchers at NGK Corporation.
DDR membranes were first reported in 2004 [23] by Tomita et al, who used a seeded-growth method [24]. Large DDR crystals were first synthesized and ground into smaller ‘seed’ particles, which were dispersed on a α-alumina tubular support by immersing it into a seed particle suspension. The seeded tubes were then used to grow a DDR membrane by hydrothermal treatment. For equimolar CO2/CH4 binary mixtures, the CO2 selectivity and permeance were only 220 and 7×10-8 mol/m2.s.Pa (209 GPU) at 301 K and a feed pressure of 0.5 MPa.
U.S. Pat. No. 6,953,493 and U.S. Pat. No. 7,014,680 by NGK Corp. teach the basic method of forming DDR zeolite membranes by mixing seed crystal into a growth solution, applying to the porous substrate, and growing a membrane thereon by hydrothermal synthesis. The seed crystals used therein were produced by grinding to 5 um, and thus do not have a regular, repeatable morphology, nor can it enter the pores of most porous substrates, thus weakening the resulting membrane. Thus, the use of ground material as seed crystals is clearly less than optimal.
US2009011926, also by NGK Corp., teaches a similar method. Here, a 300 nm seed crystal can be either dispersed in the growth solution or previously applied to the pores of the substrate. However, the seed crystals were prepared by pulverizing crystals, and thus although smaller, still suffer from the lack of uniform morphology.
Himeno et al further investigated the membrane performance at higher pressure and with impurities present in the feed stream [18]. The membrane had a CO2 selectivity and permeance of 80 and 1.1×10-7 mol/m2.s.Pa (329 GPU) at 298 K and 3 MPa feed pressure. While the permeance of CH4 was not affected by water vapor, the presence of 3% water in the gas stream reduced the CO2 permeance to half of that for a dry feed stream, resulting in a 50% reduction in the CO2/CH4 selectivity. Other impurities such as N2 and n-C3H8 had negligible effects on the performance.
US2010144512, also by NGK Corp., teaches a method for producing a DDR type zeolite membrane by immersing a porous substrate having a DDR type zeolite seed crystal adhered thereon, in growth solution to grow the membrane, and a burning step of heating the precursor at 400 DEG C. or above and at 550 DEG C. or below to burn and remove the 1-adamantaneamine contained in the DDR type zeolite membrane. However, as above, the seed crystal is produced by grinding and suffers the same disadvantages.
US2010298115 by NGK offers another method. The method forms DDR zeolite membrane containing 1-adamanthanamine on a surface of the porous substrate by subjecting a DDR zeolite to hydrothermal synthesis in the presence of DDR zeolite seed crystals, applying a glass paste onto the surface of the porous substrate so as to contact the membrane, and heating the membrane to burn away the 1-adamanthanamine contained in the membrane and melting the glass paste to form a membrane-like glass seal contacting the membrane on the surface of the porous substrate. Again, the seed crystals used herein are produced by pulverizing.
US2011160039 by NGK teach a method including forming a surface layer by attaching a low polar polymer on a first surface of a porous substrate to cover the surface, a filling step for filling a masking polymer into pores in the porous substrate from a surface different from the first surface of the porous substrate up to the surface layer by impregnating the porous substrate with the masking polymer and solidifying the masking polymer, and a surface layer removing step for removing the surface layer. After the surface layer removing step, a zeolite membrane is formed on the first surface of the porous substrate. The seed crystal is again produced by grinding, but is preferably small enough to enter the pores of the support.
However, there is no prior art on the synthesis of nanosized (i.e., sub-micron) DDR crystals of uniform shape and size, which are critical in fabricating high-quality membranes. In this disclosure, methods for synthesizing nanometer to micron size DDR zeolite crystals are described. Not only can the size and shape of the DDR crystal be controlled, but the synthesis time is significantly shortened. Thus, the methods and compositions described herein are a significant improvement on the prior art. Furthermore, membranes made with such crystals are much improved over the prior art membranes.